The dog playing Laddie (the son of Lassie) is actually Pal - the dog who played the original Lassie in La cadena invisible (1943).
This was the only Lassie movie in which June Lockhart starred. However, 13 years later, she would appear in her first of 207 episodes of the Lassie (1954) television series.
This film was the first shot in the Technicolor "monopack" process, where one magazine of film registered all three primary colors, rather than the original three-strip Technicolor process (introduced in 1932), where a separate magazine of film had to be exposed (and processed) for each of the three primary colors.
Nils Asther (Olav) had been a popular leading man in silent movies. Brought to the U.S. from his native Sweden by M-G-M, he costarred with the likes of Pola Negri, Joan Crawford and fellow Swede Greta Garbo, to whom he proposed (but was rejected).
By the time he made this, Nigel Bruce was widely known for his recurring role as Dr. Watson to Basil Rathbone's Sherlock Holmes in Universal's popular detective movies, of which more than a dozen were made.